Fluid suspension systems for vehicles are well known and usually include one or more pneumatic springs at each end of each axle whereby the sprung mass of the vehicle (the chassis, body and load) is supported on the unsprung mass (the wheels, suspension, and axles). When dealing with trailers, large trucks or buses it is desirable to maintain the clearance height of the vehicle bed fixed relative to the axle even if the vehicle is not horizontal and the load therein is not centered. It is well known to provide automatic levelling systems which operate to maintain the clearance height. Such systems may include, for each set of air bags, a regulating valve having an arm pivoted thereto and a fixed-length link extending between the arm and the axle. As one end of the axle moves downwardly relative to the vehicle bed, for example, when the vehicle tilts, the link moves downwardly as does the arm on the regulating valve. Such movement of the arm causes the valve to bleed pressurized air from the air bags to effectively lower the vehicle bed to be generally parallel to the axle and hence compensate for downward movement of the axle. Similarly, upward movement of one end of the axle results in increased pressurization of the air bags to effectively raise the vehicle bed to be generally parallel. These alterations in clearance height between the sprung and unsprung masses tend to maintain the sprung mass generally parallel to the unsprung mass even though the unsprung mass moves upwardly or downwardly through a horizontal plane.
It is also desirable to be able to selectively alter the clearance height between the sprung and unsprung masses when the vehicle is stationary, as when a trailer is positioned at a loading dock. A loaded trailer may be lower than the level of the dock and this can hinder the movement of cargo from the vehicle bed to the dock, especially when fork lift trucks are used to remove cargo or when a conveyor may be used to take cargo directly from the vehicle to the dock or vice versa.
This idea is not new in the art, as examplified by U.S. Pat. No. 3,074,739 issued Jan. 22, 1963 to G. Alfieri. In the Alfieri invention a spring biased pneumatic cylinder replaces the link in the automatic systems and may be pressurized at will to override the automatic system. The Alfieri system is complex, expensive and relies on springs and compressed air to operate. Such use reduces the precision control required. This system does not provide tilting control features or feedback control.